The invention relates to wavelength control and stabilisation in tunable devices such as lasers, filters and the like.
Proper stabilisation of the operating wavelength over a range corresponding to several channels of a WDM (Wavelength Domain Multiplexing) system is an essential requirement for components intended to be used in WDM systems, especially of the xe2x80x9cdensexe2x80x9d type (DWDM).
Preferably, stabilisation must be independent of the source of perturbation, should be achieved at low cost and must be compatible with high level integration to give rise to laser sources of reduced dimensions.
A traditional arrangement resorted to for wavelength stabilisation of laser sources is based on gas spectral line absorption as a reference. While providing a high degree of accuracy, such a solution is not suitable for integration and is associated with inherently high costs.
Alternative solutions provide for the use of optical couplers and optical filters (see, for instance, U.S. Pat. No. 5,781,572), discrete photodiodes/filters with additional components (see for instance U.S. Pat. No. 5,825,792) and/or holographic/diffraction gratings. Other documents such as U.S. Pat. No. 6,134,253, U.S. Pat. No. 6,144,025 or U.S. Pat. No. 6,215,801 are of interest in the area of the invention.
A double diode consisting of a counter-polarised Si-Schottky diode and a SiGe PIN diode is known per se from WO-A-98/56045 where use for optical converters, logical switched networks as well as solar cells is contemplated.
Also, in European Application No. 01300426.6xe2x80x94which forms part of the prior art under the provisions of Art. 54(3) EPCxe2x80x94a device for monitoring the emission wavelength of a laser is disclosed including a wavelength selective filter exposed to the radiation emitted by the laser source. The device includes a semiconductor slice having first and second opposed surfaces. The first surface of the slice is exposed to the radiation at an angle to the main emission axis of the source such that a portion of the radiation impinges onto the first surface at angles in the vicinity of the Brewster angle. The radiation is thus refracted into the semiconductor slice towards the second surface of the slice, where the wavelength selective filter is arranged.
The object of the present invention is to provide a device for stabilising the operating wavelength of a component such as e.g. a laser or a tunable filter which is further improved with respect to the prior art arrangements especially in terms of low manufacturing cost and high degree of integration.
According to the present invention that object is achieved by means of a device having the features set forth in the annexed claims.
Specifically, the arrangement of the invention provides for the use of a semiconductor photodiode adapted for use in combination with a processing block which controls the operating wavelength of an associated component by a proper mechanism. Exemplary of such a mechanism are i.e. controlling the junction temperature of a laser diode (for instance by means of a thermal condition device such as a Peltier cell), controlling the injection current into the proper sections of a Distributed Bragg Reflector (DBR) laser, inducing a mechanical stress in a component, varying the position of a mirror or the electrical field in an external cavity laser or a tuneable filter, etc. A wavelength conditioning action is therefore carried out by an actuator, which effectively counters any wavelength variation in the device.
Preferably, the photodiode is a double diode monolithically grown with a sequence of layers (n-i-p-i-n) corresponding to two opposed diodes.
The proposed device is able to discriminate wavelength variations in a selected spectral range as resulting from the choice of the materials included in the device structure.
The operating range can thus be adjusted in order to select the proper wavelength range and improve selectivity. This feature represents a major advance over prior art solutions such as disclosed e.g. in WO-A-98/56045 in that it provides an additional degree of freedom in bias voltage control for the definition of the zero output current wavelength. The proposed device is particularly suitable for use in the conventional and extended WDM bands (S, C, L).
The composition of the two active (i) layers is selected in such a way to provide two different photoresponses and thus two different photocurrents for any given wavelength of the laser diode. Since the photodiodes are opposed, the photocurrents subtract from each other.
Any change of the wavelength of the impinging radiation gives rise to a signal that can be processed to generate control signals for the actuator.